Physiological and biochemical mechanisms of salinity tolerance in Indian cashew (Anacardium occidentale L.)

Salinity is a major environmental stress that threatens sustainable agricultural production worldwide. Screening cashew varieties for salt tolerance is crucial for advancing breeding strategies that may guarantee sustainable cashew production under saline soils. In screening studies, finding important traits among a variety of morphological, physiological, and biochemical parameters can enhance selection efficiency and accelerate the breeding process. In this study, six cashew varieties were evaluated for salinity stress response at the seedling stage under greenhouse condition. The treatments consisted of non-saline [0 mM sodium chloride (NaCl)] and saline (50, 100, 150 and 250 mM NaCl) irrigation water under Indian conditions. A broad spectrum of morpho-physiological, ionic, and biochemical markers, along with multivariate analysis, was used to assess the salinity tolerance of the cashew varieties. Salt stress significantly reduced growth-related parameters, relative water content (RWC) and leaf pigments across most of the cashew varieties. Notably, varieties Bhaskara and Vengurla-4 exhibited less reduction in morpho-physiological traits, lower sodium (Na⁺) content and a higher salinity tolerance index. The reduction in plant biomass corresponded to 43.1 % in Bhaskara (salt tolerant variety) against 52.7 % in Madakkathara-2 (salt sensitive variety) at higher salinity. Further, Bhaskara displayed greater leaf (3.01 % dry weight) and root (2.01 % dry weight) potassium (K⁺) content and K⁺/Na⁺ ratio (1.70 in leaf and 1.57 in root) under salt stress. Salt tolerant varieties exhibited elevated proline content and higher soluble sugar content in contrast to salt-sensitive varieties. The findings were further validated through principal component, heat map and correlation analysis. K⁺ content, K⁺/Na⁺ratio, proline content and soluble sugar content emerged as the most reliable traits associated with salt-tolerance in cashew under saline conditions. The identified salt-tolerant varieties present valuable genetic resources for advancing salt tolerance breeding programs in cashew.